Shielded transformers



J. A. ROSS SHIELDED TRANSFORMERS April 4, 1967 2 Sheets-Sheet 1 FiledDec. 50, 1963 INVENTOR. JAM ES A. ROSS Jr A T TORNEV April 4, 1967 J,Ross 3,312,919

SHIELDED TRANSFORMERS Filed Dec. 50, 1963 2 Sheets-Sheet 2 INVENTOR.

JAMES A. ROSS A TTOPNE V United States Patent Filed Dec. 30, 1963, Ser.N0.I334,219

2 Claims. c1. 336-84) The invention relates to shielded transformers,more particularly to so-called miniature transformers, and to the methodof fabricating such shielded transformers.

The present day sophistication of electrical and electronic instrumentsand components has led to many attempts to reduce the size of includedparts so that the multiplicity of parts does not result in impracticalbulk. Heretofore the so-called miniaturization of shielded transformershas been limited by the irreducible thicknesses of the variousinsulating and shielding materials.

Recent chemical advances have made possible the development ofinsulating materials which may be applied by spraying or taping theinvolved electric components. New liquid conductive materials, such asthe E. I. du Pont de Nemours & Company air dry silver preparations, haveadded great flexibility to the design of conductive patterns.

My invention utilizes these new advances in the field of materials andresults in shielded transformers of ap preciably less bulk thanheretofore possible. The invention contemplates a shieldedxtransformerthat comprises a first winding of conductive wire about an insulatedcore and a first insulating layer about the first winding. A firstshielding coating about the first insulated winding has separatedcoating edges defining a coating gap. The coating is preferably of adried liquid conductive material, such as that mentioned above, that isotherwise continuous about the insulated winding. A first insulatingstrip covers the gap between the edges of the shielding coating. Thestrip overlaps the coating on either side of the edges so that a secondcoating of a dried liquid conductive material on the insulating stripjoins one edge of the first coating and overlies but does not join thesecond edge of the first shielding coating. A foil patch having aconductive lead is fixed to the outer surface of the coating. A secondinsulating layer covers the shielding coatings and the foil patch, butpermits the conductive lead to extend exteriorly. A second winding ofconductive wire about the second insulating layer has a third insulatinglayer about it. A second thin shielding coating of dried conductiveliquid and having separated edges defining a coating gap covers theouter surface of the third insulating layer. A second insulating stripoverlies'the interval between the separated edges of the secondshielding coating and has a conductive coating applied thereon in themanner described for the first insulating strip. A second foil patchwith a conductive lead is fixed to the shielding coating and a fourthinsulating layer covers the conductive coatings with the electrical leadextending from the foil patch externally. Means may also be provided forexternal connection with the first and the second conductive windings.The insulated transformer coil may be insulated by a molded phenoliccovering, or may be insulated from the windings by a bobbin of asuitable insulating material.

The insulating layers may be of dielectric tape, sprayed coatings, ordipped solutions.

The shielding coating obviously can be applied in a very thin layer byconventional painting methods. Because ma terial need not be folded oroverlapped to conform to shape, the bulk of the shielding isconsiderably reduced. This is particularly true of toroidal transformerswherein the warped surfaces do not lend themselves to linear wrapping.

The invention includes a process for fabricating a 3,312,919 PatentedApr. 4, 1967 ice shielded transformer such as that defined above. Theprocess comprises the steps of winding conductive wire about aninsulated transformer core and applying a first layer of insulationmaterial completely about the windings. Such layer may be applied bywrapping, dipping or spraying. An area of the insulation surface ismasked and a thin coating of liquid conductive material applied over theexposed surfaces of the first layer of insulation material and themasked area. The liquid conductive material is then dried, forming aconductive shielding about the insulated first winding. The coatedmasking is removed to break the continuity of the shielding coating. Asecond layer of insulating material is applied to the unshielded arealeft by the removal of the masking. The second layer extends beyond eachedge of the uncoated first layer of insulation. A conductive foil patchwith a wire lead is applied to the coated surface after the conductivematerial is dried. A second thin coating of liquid conductive materialis applied to a portion of the second insulating layer so that thesecond coating makes contact with the applied conductive coating at oneedge of the second insulating layer without making contact at the otheredge of the conductive coating. The two applied coatings of conductivematerial are such that the entire outer periphery of the first insulatedlayer is surrounded and thus shielded by an electrically discontinuousconductive shield. The second liquid conductive coating is then driedand an insulating layer is applied about the entire shielded surface.

The method and apparatus described can be utilized for, and result in,miniature transformers of many configurations. The invention hassuccessfully been utilized with transformers of conventional toroidal,U, C, E, I, cup and L-shaped transformer cores. Each transformer,regardless of its configuration, may be made smaller than conventionallybecause of the invention. Much production time is also saved by theinventive fabricating process described.

These and other advantages of the invention are apparent in thefollowing detailed description and drawing in which:

FIG. 1 is a schematic perspective view of a conventional E-shapedtransformer having a center winding in accordance with the invention;

FIG. 2 is a side elevation of the embodiment of FIG. 1, partly brokenaway;

FIG. 3 is a perspective view of an insulated wire bobbin for use on atransformer core;

FIGS. 4-7 are side elevational views of the bobbin of FIG. 3illustrating the steps of electrostatically shielding the bobbin;

FIG. 8 is a fragmentary sectional elevation of the transformer of FIG.1, taken along line 88 of FIG. 1;

FIG. 9 is a perspective view of a shielded toroidal transformer inaccordance with the invention; and

FIG. 10 is an enlarged sectional elevation taken along line 10-10 ofFIG. 9.

In FIG. 1 a transformer 10 in accordance with the invention has theconventional E configuration core 11. The core may be a laminar core.About the central post 11A, of the core are the insulated andelectrostatically shielded windings 12 from which electrical leads, suchas the wires 14, 15, 16 extend. An outer insulating layer 18 surroundsthe windings 12, and in FIG. 1, conceals the components of the windings.The windings are about a bobbin 19, preferably of dielectric plasticmaterial. Bobbin 19 is shown in more detail in FIG. 3.

The bobbin has a hollow shank 21 terminating at opposite ends in flanges22, 23. A central aperture 25 extends through the shank of the bobbin.Aperture 25 conforms to the general exterior configuration of centralpost 11A of the transformer core.

In FIG. 2 the bobbin without its electrical windings is shown in placeabout post 11A of transformer core 10. Lower flange 23 of the bobbinrests upon a back 26 of the E-shaped core. The bobbin has anelectrostatic shielding coating 28 that covers shank 21 of the bobbinand the inner surfaces of flanges 22 and 23. This covering of shieldingconductive material is interrupted by a continuous shielding gap 29. Asshown in FIG. 2, the gap is continuous from outer edge 31 of flange 23-and across the flange, the shank, and flange 22 to the outer edge 33thereof.

The conductive electrostatic shielding coating is preferably applied asa liquid by dipping, spraying, or by brush and then dried to a smooth,uniformly thin layer. A small conductive foil patch 35, having aconductive wire lead 36 secured thereto, is fixed to the bobbin byadhering it to some convenient portion of the shielding coating. Aninsulating strip 38, which is partly broken away in FIG. 2, overlies gap29 and overlaps the edges 41, 42 of the shielding coating. A secondaryshielding coating 28A covers all but a small border area 43 of strip 38.

The method whereby the bobbin is shielded is illustrated in FIGS.4through 7. In FIG. 4 the bobbin 19 has applied thereto a narrowelongate masking strip 45 which extends across bobbin shank 21 andoutwardly from the shank across both flanges 22 and 23 to break theelectrical continuity of conductive shielding coating 28.

In FIG. the entire unmasked outer surface of the shank and innersurfaces of the flanges are covered with electrostatic shielding coating28, as described above. The masking strip 45 is also covered to makeless exacting the task of applying the liquid conductive coating to thebobbin.

After the liquid coating has been dried, the masking strip 45 iscarefully removed. The bobbin then has a discontinuous electrostaticshielding coating 28 having a gap 29 defined by the aforesaid coatingedges 41, 42, as shown in FIG. 6. In FIG. 7 the shielded bobbin hasapplied thereto a first insulating strip 47 that adheres to theshielding coating on both the shank and the flanges of the bobbin andextends slightly beyond edges 41 and 42 on both the shank and theflanges. The conductive foil patch 35 and its lead 36 may be applied tothe bobbin at the same stage as the application of the first insulatingstrip. A thin added layer 28A of liquid conductive material is thenapplied to the insulating strip 47, making electrical contact with thedried shielding coating 28 adjacent edge 41. Note that the added liquidcoating does not extend beyond border 51 of the first insulating strip46 and is therefore out of contact with the first applied shieldingcoating adjacent the border, but overlies the shielding coatingimmediately adjacent edge 42 of gap 29. The adding layer may also beextended to include the foil patch to insure perfect contact.

After the bobbin has been prepared as illustrated in FIGS. 47,conventional transformer windings 53 are wound about the shank of thebobbin over an insulating layer 54 that is applied about the shieldingcoating 28 on the bobbin. After the requisite number of turns arewrapped about the bobbin, a second insulating layer 155 is applied overthe winding. A second masking strip is then applied to the insulatinglayer 55 and the exposed insulating layer is coated with a thin liquidshielding material, encompassing the masking strip. After the conductivecoating 56 is dried, the masking strip is removed to make discontinuousthe conductive path of the electrostatic shielding coating about theinsulating layer. A second insulating strip and a second lead-bearingconductive foil patch 57 are then applied to the dried shielding coatingin the manner described previously with respect to bobbin 19. The secondinsulating strip is then coated with a shielding coating, as previouslydescribed, to complete the electrically discontinuous shielding coatingabout the entire insulating layer.

The shielding coating is covered by a third insulating layer 58 aboutwhich second transformer windings are wound. These second windings arethen covered by an insulating layer 59 shielded in the manner previouslydescribed by a shielding coating 60 which is covered by an insulatinglayer 61.

Since the objective of the method and apparatus of the invention is toachieve the most compact transformer unit possible, it is preferablethat the insulating layers employed in the combination also be as thinas electrically possible. The insulating layers may be thin dielectricape wrapped about the components of the transformer 31 may be sprayed ordipped coatings that harden into a dielectric layer.

In accordance with conventional practice, each electrostatic shieldingcoating has an electrical lead 36 extending externally of the lastinsulating layer, as do the leads from the windings themselves extend.Conventional electrical circuitry is employed to connect the transformerof the invention in the desired circuit. The method described aboveresults in a transformer with much less bulk and weight thanconventional transformers capable of performing the same functions. Inaddition to a saving in size and weight, the use of fast drying liquidelements lessens the material and labor costs usually necessary toproduce transformers of the same quality. While the transformerillustrated has been one of conventional E-core configuration, theinvention is equally applicable to transformer of C, U, E1 or toroidalcore configurations. J

A toroidal transformer 65 is illustrated in FIGS. 9 and 10. Transformer65 has in insulated core 66 of toroidal configuration. conventionallythe core is of silicone steel and may have a phenolic insulating molding67. A first winding 68 of conductive wire surrounds the core. A firstinsulating layer 69 covers the first windings.

While wrapped windings of insulated ta-pe are used in the illustrativeembodiment, other suitable insulating layers may be used, such as fastdrying liquid coatings applied by dipping or spraying.

A first thin electrostatic shielding coating 71 forms a discontinuouscoating about the insulating layer 69. A conductive gap 72 defined byterminal edges 71A, 71B of the shielding coating (see FIG. 10) precludeselectrical shorting of the shielding coating. A masking strip 73 havinga shielding coating 71C over the major portion of its exterior completesthe shielding about the first winding. The masking strip coating iselectrically connected only to edge 71A while overlapping edge 71B.

A second insulating layer 74 completely covers the shielding coating 72.A second discontinuous electrostatic shielding coating 75 surrounds thesecond insulating layer 74. The second coating 75, while electricallydiscontinuous, completely surrounds the insulating layer by combiningwith a coated mask similar to the masking strip 73 described withrespect to first shielding coating 71.

A third insulating layer 77 completely covers the second shieldingcoating 75. A second winding 78 of conductive wire is wound upon theinsulating layer in spiral fashion about the toroidal shape. The secondwinding is covered by a fourth insulating layer 79, in turn surroundedby a third discontinuous electrostatic shielding coating 81. The entiretoroidal shielded transformer thus far described is covered by a finalinsulating layer 83. All of the shielding coatings are of a dried liquidzonductive material.

Leads from each of the first and second windings, and from theconductive electrostatic shielding coatings extend externally of thetransformer. Such leads may be fixed to the shielding coatings in themanner described for the embodiment illustrated by FIGS. 1-7. Theshielded toroidal transformer shown in FIGS. 9 and 10 is extremelycompact, yet very efficient electrically because of the doubly shieldedwindings.

The method whereby the described shielded toroidal transformer isfabricated is as follows: the first transformer windings are wrappedabout the insulated core. The windings are then covered with aninsulating layer such as insulating tape. A first narrow masking band isplaced on the insulating tape in a complete circle about the outside ofthe toroid. A thin conductive liquid coating is then applied over theinsulating tape and by the mask by spraying, dipping or brushing. Thecoating is dried and the masking band peeled from the insulating tape,leaving a nonconductive gap defined by the terminal edges of theconductive coating. An insulating strip is placed over the uncoatedarea'left by the removal of the masking strip. The insulating stripoverlaps the coating at each edge completely around the toroid. A liquidconductive coating is then applied by brushing or spraying to the majorpart of the insulating strip, contacting the previous conductive coatingnear one of its edges and overlying the second edge of the previousconductive coating without making electrical contact therewith. Thesecond coating is dried. The entire t-oroid is surrounded by theconductive electrostatic shielding coating but the coating is anincomplete conductive path. A foil patch having a lead-out wire is thenadhered to the shielding coating. It is preferable that the patch iscovered with additional liquid conductive coating at the same time thatthe insulating strip is coated, to insure good electrical contact.

A second insulating layer is applied to the now shielded toroid. Asecond-transformer winding of conductive wire is wrapped about theinsulating layer. The second winding is then insulated, and thenshielded by a further discontinuous conductive electrostatic shieldingcoating applied in liquid form to the insulating layer in the mannerpreviously described. A final insulating layer is placed about theinsulated and shielded toroidal transformer windings after a foil patchwith a lead-out wire is fixed to the shielding coating. The externalwires for the transformer windings are of course made externallyaccessible through the insulating and shielding layers.

Variations of method and apparatus within the scope of the inventionwill occur to those skilled in the art. While standard transformerconfigurations have been referred to illustratively, the invention isapplicable to a variety of transformer and transformer core shapes.Split windings and multiple transformer stage windings are equallywithin the scope of the invention. Therefore, I wish that the inventionbe measured by the appended claims rather than by the purelyillustrative embodiments disclosed herein.

I claim:

1. An electrostatically shielded transformer comprising an insulatedtransformer core, a first winding of conductive wire about the insulatedcore, a first insulating layer about the first winding, a firstelectrostatic shielding coating about the insulated first winding, acoating ga-p defined by separated'coating edges, said coating being of adried liquid conductive material continuous between its separated edges,a first insulating strip more than spanning the gap between the edges ofthe shielding coating, an added coating of a dried liquid conductivematerial on the insulating strip such that the added coatingelectrically joins one edge of the first coating and overlies but doesnot electrically join the second edge of the first shielding coating, aconductive lead extending externally from the shielding coating, asecond insulating layer covering the electrostatic shielding coatings, asecond wind.-

ing of conductive wire about the second insulating layer, a thirdinsulating layer about the second conductive windings, a secondelectrostatic shielding coating of dried conductive liquid havingseparated edges defining a coating gap and applied to the outer surfaceof the third insulating layer so that the coating is continuous be tweenits separated edges, a second insulating strip overlying the intervalbetween separated edges of the second shielding coating and extendingover the coating on each side of the separated edges, an addedconductive coating on the second insulating strip electrically joiningone edge but electrically separate from and overlying the other edge, anelectrical lead fixed to the second shielding coating, a fourthinsulating layer covering the conductive coatings such that theelectrical lead extends externally, and means for external connectionsto the first and the second conductive windings.

2. An electrostatically shielded transformer comprising an insulatedtransformer core, a first winding of conductive wire about the insulatedcore, a first insulating layer about the first winding, a first thinelectrostatic shielding coating about the insulated first winding,separated coating edges defining a narrow coating gap making the coatingelectrically discontinuous, said coating being of a dried liquidconductive material continuous between its separated edges, a firstinsulating strip spanning the gap between the edges of the shieldingcoating and fixed to the coating, an added coating of a dried liquidconductive material on the insulating strip such that the added coatingelectrically joins one edge of the first coating and overlies but doesnot electrically join the second edge of the first shielding coating, afoil patch having a conductive lead extending therefrom fixed to theshielding coating, a second insulating layer covering the electrostaticshielding coating and the foil patch so that the conductive lead extendsexteriorily therefrom, a second winding of conductive wire about thesecond insulating layer, a third insulating layer about the secondconductive windings, a second thin electrostatic shielding coating ofdried conductive liquid having separated edges defining a coating gapand applied to the outer surface of the third insulating layer so thatthe coating is continuous between its separated edges, a secondinsulating strip overlying the interval between separated edges of thesecond shielding coating and extending over the coating on each side ofthe separated edges, an added conductive coating on the secondinsulating strip electrically joining one edge but electrically separatefrom and overlying the other edge, a second foil patch having anelectrical lead and fixed to the shielding coating, a fourth insulatinglayer covering the conductive coatings such that the electricalleadextends externally from the foil patch, and means for externalconnections to the first and the second conductive windings.

References Cited by the Examiner UNITED STATES PATENTS 2,914,719 11/1959Walton et a1. 336--84 X 3,063,135 11/1962 Clark 33644 x 3,143,720 8/1964Rogers 336-429 X 3,201,854 8/1965 Cox 29 1ss.s7

LEWIS H. MYERS, Primary Examiner. L. E. ASKIN, Examiner. T, J. KOZMA,Assistant Examiner.

1. AN ELECTROSTATICALLY SHIELDED TRANSFORMER COMPRISING AN INSULATEDTRANSFORMER CORE, A FIRST WINDING OF CONDUCTIVE WIRE ABOUT THE INSULATEDCORE, A FIRST INSULATING LAYER ABOUT THE FIRST WINDING, A FIRSTELECTROSTATIC SHIELDING COATING ABOUT THE INSULATED FIRST WINDING, ACOATING GAP DEFINED BY SEPARATED COATING EDGES, SAID COATING BEING OF ADRIED LIQUID CONDUCTIVE MATERIAL CONTINUOUS BETWEEN ITS SEPARATED EDGES,A FIRST INSULATING STRIP MORE THAN SPANNING THE GAP BETWEEN THE EDGES OFTHE SHIELDING COATING, AN ADDED COATING OF A DRIED LIQUID CONDUCTIVEMATERIAL ON THE INSULATING STRIP SUCH THAT THE ADDED COATINGELECTRICALLY JOINS ONE EDGE OF THE FIRST COATING AND OVERLIES BUT DOESNOT ELECTRICALLY JOIN THE SECOND EDGE OF THE FIRST SHIELDING COATING, ACONDUCTIVE LEAD EXTENDING EXTERNALLY FROM THE SHIELDING COATING, ASECOND INSULATING LAYER COVERING THE ELECTROSTATIC SHIELDING COATINGS, ASECOND WINDING OF CONDUCTIVE WIRE ABOUT THE SECOND INSULATING LAYER, ATHIRD INSULATING LAYER ABOUT THE SECOND CONDUCTIVE WINDINGS, A SECONDELECTROSTATIC SHIELDING COATING OF DRIED CONDUCTIVE LIQUID HAVINGSEPARATED EDGES DEFINING A COATING GAP AND APPLIED TO THE OUTER SURFACEOF THE THIRD INSULATING LAYER SO THAT THE COATING IS CONTINUOUS BETWEENITS SEPARATED EDGES, A SECOND INSULATING STRIP OVERLYING THE INTERVALBETWEEN SEPARATED EDGES OF THE SECOND SHIELDING COATING AND EXTENDINGOVER THE COATING ON EACH SIDE OF THE SEPARATED EDGES, AN ADDEDCONDUCTIVE COATING ON THE SECOND INSULATING STRIP ELECTRICALLY JOININGONE EDGE BUT ELECTRICALLY SEPARATE FROM AND OVERLYING THE OTHER EDGE, ANELECTRICAL LEAD FIXED TO THE SECOND SHIELDING COATING, A FOURTHINSULATING LAYER COVERING THE CONDUCTIVE COATINGS SUCH THAT THEELECTRICAL LEAD EXTENDS EXTERNALLY, AND MEANS FOR EXTERNAL CONNECTIONSTO THE FIRST AND THE SECOND CONDUCTIVE WINDINGS.